The invention concerns a roller hearth furnace for the heat treatment of parts passing therethrough. The furnace includes a thermally insulated housing forming a heating space therein. A plurality of rotatable rollers are mounted in the heated spaced and extend in a direction transversely to the direction of travel of parts through the heating space. The parts ride on the rollers, and the rollers are rotatably driven to transport the parts.
A roller hearth furnace of that type is known, for example, from German Patent DE-28 40 282. In that known configuration refractory bricks are built into the refractory lining of the opposing longitudinal walls of the roller hearth furnace, and are provided with transversely aligned openings to contain the rollers. The rollers can then be inserted into the aligned openings from outside of the furnace and removed in a similar manner, if repairs are necessary.
In order to prevent the loss of gas and heat through the openings in the refractory bricks, the diameter of which must be greater than that of the rollers, cavities are provided in the refractory bricks, into which an insulating and refractory material is pressed, such as for example kaolin wool, which may then contact the roll periphery tightly.
A fundamental disadvantage of such roller hearth configurations results from the fact that for reasons of mechanical strength the diameter of the rollers themselves cannot be reduced below a certain value and the spacing between the rollers may also not be arbitrarily small. Thus, the openings in the longitudinal walls of the furnace constitute a considerable weakening of the walls, i.e., relatively narrow web portions of the walls remain in between the openings which must withstand the various loads such as the outwardly directed tensile stress under which the rollers are constantly placed. To maximize the strength of those webs, they can be provided with a certain minimum width, but then a corresponding spacing between the rollers will also result. This spacing between the rollers and their diameter determines the size of the parts to be treated (to prevent the parts from falling between the rollers) unless separate carrier plates for the parts are provided. The disadvantage in using plates involves the fact that the mass of the carrier plates must also be heated during their travel through the roller hearth furnace, thereby causing an unnecessary loss of heat.
In other types of industrial furnaces, for example the so-called tunnel furnaces (DE 35 10 801), in which the parts to be treated are moved on transport elements through the furnace, the space problem present in roller hearth furnaces does not arise, as no space is needed for the changing of the rolls adjacent to the furnace. However, the disadvantage exists that the transport elements must be heated during their passage through the furnace. This fact is not altered by the provision of rotating rollers in lateral spacers either. These elements cannot be used for roller hearth furnaces, in which the rollers are stationary, because in view of the guides for the grate-like transport elements, which are mounted exclusively inside the furnace, the stationary rollers could not be exchanged.
It is, therefore, an object of the invention to provide a roller hearth furnace of the above-mentioned type such that the spacing between the rollers may be minimized while using the smallest possible roller diameters, without causing stability problems in the furnace walls or difficulties relative to the replacement of the rollers.